1. Field of the Invention
The present invention relates to atmospheric ionization of analytes and mass spectrographic methods.
2. Description of Related Art
Regulatory and safety issues related to the use of radioactive materials, such as 63Ni, 241Am, and 3H, among others, have led to a search for non-radioactive ion sources for analytical instruments, such as ion mobility spectrometers. (See Turner et al. U.S. Pat. No. 6,225,623 entitled “Corona Discharge Ion Source for Analytical Instruments”, Doring U.S. Patent Application Publication No. 2002/0185593 entitled “Ion Mobility Spectrometer with Non-Radioactive Ion Source”.), and Kojiro et al., “Determination of C1–C4 Alkanes by Ion Mobility Spectrometry”, Anal. Chem., 1991, 63, 2295–2300.
Certain available corona discharge ion sources for atmospheric pressure ionization (API) mass spectrometers or ion mobility spectrometers (IMS) or chemical agent monitors (CAM) introduce the analyte (including solvent, air, and other contaminants) into the region containing a discharge needle. This leads to several problems:
1. The presence of oxygen or other contaminants in the air leads to degradation of the electrodes.
2. It can be difficult to maintain the discharge in the presence of contaminants, requiring a high electrical potential or pulsed potentials.
3. A corona discharge in air leads to the formation of species, such as NO2−, NO3−, and related cluster ions. These ions can cause a loss of sensitivity for analyte ions (C. A. Hill and C. L. P. Thomas, Analyst, 2003, 128, pp. 55–60) and can interfere with the detection of NO2− and NO3− produced from analytes containing nitro functional groups, such as nitro explosives or in the case of chloride ion interference with chlorate propellants for rocket motors or phosphate interference with chemical warfare-related compounds. Attachment of these mixed nitric oxides to explosive molecules will misrepresent the degree of nitration of the explosive. This is a serious drawback because the scale of explosive power is related to the degree of nitration on the molecule.
4. Introducing air and analyte into the discharge region limits the possibilities for controlling the nature of the chemical background to control the ion-formation chemistry.
Taylor et al. U.S. Pat. No. 5,684,300 entitled “Corona Discharge Ionization Source” and Turner et al. U.S. Pat. No. 6,225,623 B1 entitled “Corona Discharge Ion Source for Analytical Instruments” describe corona discharge ion sources, but do not describe separating the region where the discharge occurs from the region where the analyte is introduced. See also Zhao et al. entitled “Liquid Sample Injection Using Atmospheric Pressure Direct Current Glow Discharge Ionization Source,” Anal. Chem., 64, pp. 1426–1433, 1992.
Tsuchiya et al. U.S. Pat. No. 4,546,253 entitled “Apparatus for Producing Sample Ions” describes a method for using metastable atoms to produce ions from a sample introduced at the tip of an emitter needle downstream from the corona discharge. This technique requires that the sample be placed on or near an intense electric field emitter needle. See also Otsuka et al. entitled “An Interface for Liquid Chromatograph/Liquid Ionization Mass Spectrometer,” Analytical Sciences, Vol. 4, October 1988. Tsuchiya has developed a technique called “Liquid Surface Penning Ionization” (LPI) that uses excited-state argon atoms produced in a corona discharge to ionize liquid samples. The liquid samples are deposited onto the tip of a heated needle at atmospheric pressure to which kilovolt electrical potentials are applied.
The present invention avoids use of an emitter needle at high electrical potential placed downstream of the corona discharge source. Further, the present invention provides a method of sampling neutral analyte molecules without the restriction of relocating the analyte from the surfaces on which they are attached. For example, cocaine from cash currency, and chemical/biological warfare agents from surfaces of military interest can be sampled directly and in situ without swabbing or solvent washing the surface. Each time sample is relocated, analyte molecules are lost (30 to 100% for trace-level concentrations). Therefore, direct surface sampling is always preferred.
Long-lived excited-state atoms and molecules (metastables) have been used in only a few other ion source designs. Bertrand et al. U.S. Pat. No. 6,124,675 entitled “Metastable Atom Bombardment Source” discloses a metastable atom source operating at reduced pressure for generating ions in a mass spectrometer. The device described requires substantially reduced pressures and does not describe means for using metastable atoms for atmospheric pressure ionization mass spectrometry or ion mobility spectrometers. The MAB source produces a beam of metastable atoms at reduced pressure that are introduced under vacuum into a conventional chemical ionization source. Analyte ions are produced by Penning ionization, a reaction between an energetic atom or molecule M* and a sample molecule S:M*+S->S++M+c−  (1)
A high degree of selectivity can be achieved by selecting species with excited-state energies just above the ionization potential of a specific analyte. Background interference with ionization potentials greater than the energy of the excited-state will not be ionized. The MAB source was tested in our laboratory and was found to be efficient and selective. However, it is limited to volatile analytes that can be introduced into a mass spectrometer vacuum system.
McLuckey and coworkers developed a technique called “Atmospheric Sampling Glow Discharge Ionization” (ASGDI) in which sample vapors are introduced through an orifice into a glow discharge at reduced pressure. McLuckey, S. A.; Glish, G. L.; Asano, K. G.; Grant, B. C., Anal. Chem., 1988, 60, 2220. “Atmospheric Sampling Glow Discharge Ionization Source for the Analysis of Trace Organics in Ambient Air.” Guzowski et al., J. Anal. At. Spectrom., 1999, 14, 1121–1127 “Development of a Direct Current Gas Sampling Glow Discharge Ionization Source for Time-of-Flight Mass Spectrometer”.
Hiraoka and coworkers recently reported atmospheric pressure Penning ionization mass spectrometry (APPeI) in which a gas stream was passed through an atmospheric pressure argon corona discharge to induce ionization prior to introduction into the atmospheric pressure interface of a time-of-flight mass spectrometer. Hiraoka, K.; Fujimaki, S.; Kambara, S.; Furuya, H.; Okazaki, S., Rapid Commun. Mass Spectrom., 2004, 18, 2323–2330. “Atmospheric-Pressure Penning Ionization Mass Spectrometry”.
All of these methods suffer from certain limitations that prevent direct analysis under ambient conditions. For example, MAB and ASGDI require that a volatile sample be introduced into a region of reduced pressure. LPI requires that the sample be placed on a heated needle that is held at high (kilovolt) electrical potentials. APPeI makes use of a ring of ten corona needles operated at kilovolt potentials surrounding the orifice into the mass spectrometer API interface. These factors hinder the ability to analyze samples directly without modification and without risk to the operator.
Furthermore, MAB, ASGDI, and APPeI introduce the sample directly into the electrical discharge region. It is well known that an electrical discharge can cause sample decomposition and unwanted reactions. In fact, corona discharge methods are used for the destruction of naval and cruse derived wastes.